The invention relates to the field of communication systems, and in particular to mitigating the effects of far-end crosstalk noise in communication systems.
The Very High-speed Digital Subscriber Line is a service that allows distribution of high data rates (currently 23 Mb/sec downstream, 3 Mb/sec upstream for asymmetrical services, 13 Mb/sec for symmetrical services) using the present twisted-pair copper infrastructure. Typically, optical fiber is used to transport data to the residential area. From there, data is transmitted over the present copper infrastructure. Standardization efforts of this technology are currently under way in US (ANSI T1E1), Europe (ETSI) and International Telecommunication Union (ITU).
Due to the large attenuation of high frequency signals on the twisted-pair lines, the deployment of VDSL is limited to a radius of less than 4500 feet from the signal source. This results in generally two possible configurations for VDSL. FIG. 1a illustrates a so-called fiber-to-exchange configuration (FTTEx) for customers 102 close to the central office (CO) 100 (within about 4500 ft). In this case, VDSL is deployed across the present twisted-pair copper infrastructure 104 from CO 100. For the rest of the customers, VDSL is deployed using a so-called fiber-to-the-cabinet configuration (FTTCab) as illustrated in FIG. 1b. In this case, optical fiber 106 is run from CO 100 to an optical network unit (ONU) 108. VDSL is then deployed to customers 102 from ONU 108 across the copper infrastructure 104.
The band allocation for upstream VDSL starts at 2.5 MHz. As a result, far-end crosstalk (FEXT) noise is the dominant crosstalk noise source. In general, telephone loops disposed adjacent to each other and carrying signals at the same frequencies often create cross-talk interference in their neighboring channels. FEXT noise is the crosstalk noise generated by signals traveling in the same direction in the adjacent loops. The FEXT noise power spectral density (PSD) in a telephone loop due to a neighboring interferer, k, is equal to:PSDFEXT=kF·f2·S(f,lk)·|H(f,lk)|2·ls
where kF=constant;                f—frequency (Hz);        S(f,lk)—transmit PSD of the interferer;        H(f,lk)—transfer function of the interferer channel;        ls—the length of the cable segment where the two signals run in parallel. For an upstream VDSL environment this is generally given by the loop length of the unit placed closer to the ONU/CO, whether interferer or interfered.        
As can be seen, the PSDFEXT depends on the frequency, the length of the cable segment where the two signals run in parallel, and the channel transfer function. Generally, the channel transfer function is an exponentially decreasing function of lk. Accordingly, in an upstream VDSL environment, an interferer located close to the ONU/CO will inject significant noise over the attenuated signal on a long loop. This is further described with reference to FIG. 2.
Generally, FIG. 2 illustrates neighboring loops injecting FEXT noise into loops on the same binder. In the upstream direction (from customer transmitter to the ONU/CO), transmitter Tx4US is much closer to ONU/CO 200 than Tx2US and Tx3US. Tx4US will inject a relatively high level of FEXT noise in the copper pairs 202 and 204 collocated in the same cable binder. The upstream signals from these pairs, 202 and 204, are heavily attenuated at the point where the FEXT noise injection occurs. The result is that, in the upstream direction, the FEXT noise from source Tx4US, close to the ONU/CO 200, will significantly degrade the SNR of the sources Tx2US and Tx3US farther away from ONU/CO 200 and collocated on the cable binder. Generally, this problem does not exist in the downstream direction (all the transmitters are located at ONU/CO 200).
The performance degradation on long loops can be mitigated by a power back-off mechanism, which reduces the transmit power on shorter loops. Several methods have been proposed for a power back-off mechanism. A method that takes into account an environment in which both FEXT and near-end crosstalk (NEXT) are present is given in C. Hwang, K. Kim, “The Analysis of a New Power Back-Off Method,” ITU contribution FI-120, February 2000. A review of the proposed methods in a FEXT dominated environment is given in Krista S. Jacobsen, “Proposal for Upstream Power Back-Off for VDSL,” ITU contribution FI-075, February 2000. Two such methods for FEXT dominated environments are the reference length method and the equalized FEXT method.
The reference length method adjusts the transmitter power such that the received signal power at the ONU/CO is equal for any loop. For the reference length power back-off method, the transmit power spectral density of an interferer k, S(f,lk), is controlled to be:
      S    ⁡          (              f        ,                  l          k                    )        =                    S        ⁡                  (                      f            ,                          l              R                                )                    ·                                              H            ⁡                          (                              f                ,                                  l                  R                                            )                                                2                                              H          ⁡                      (                          f              ,                              l                k                                      )                                      2      for a channel length lk≦lR, where lR is a reference channel length, H(f,lR) is a reference channel transfer function and S(f,lR) is a reference transmit power spectral density. The reference transmit power spectral density is typically set to the maximum allowed transmit power spectral density, and, in a VDSL environment, is typically set by the standards committee. The reference channel length is typically defined by the system operator to correspond to a target length over which the operator wishes to provide a certain level of service. Thus, for instance, the reference length may be the length of the longest loop in a binder if the operator wishes to provide the level of service to all units on the binder. Generally, if the reference length is shorter than the length of the longest loop in the binder, then all units at a distance farther than the reference length transmit with a power spectral density equal to the reference power spectral density. The reference transfer function, H(f,lR), can be calculated based on the reference loop length and the type of wire used in the system.
For the reference length method, the received signal power is identical for each loop. FEXT noise, however, is not. The longest loop is still affected more than the rest because the length of the shared portion of the cable is proportional with the loop lengths of all the other interferers, while for the shortest loop the shared portion of the cable is the length of the loop itself. The distribution of the signal to noise ratio (SNR) (and, hence, the data rate) as a function of loop length looks like that shown in FIG. 3. Thus, the data rate is considerably higher for units closer to the ONU/CO than for units that are far away.
The equalized FEXT method adjusts the transmitter power such that the FEXT noise measured at the ONU is equal for any loop, i.e. the PSD of the FEXT noise is independent of the loop length. For the equalized FEXT power back-off method, the transmit power spectral density of an interferer k is controlled to be:
      S    ⁡          (              f        ,                  l          k                    )        =                                          S            ⁡                          (                              f                ,                                  l                  R                                            )                                ·                                                                  H                ⁡                                  (                                      f                    ,                                          l                      R                                                        )                                                                    2                    ·                      l            R                                                                                            H                ⁡                                  (                                      f                    ,                                          l                      k                                                        )                                                                    2                    ·                      l            k                              ⁢                          ⁢      for      ⁢                          ⁢              l        k              ≤          l      R      For an extremely short loop in the equalized FEXT method, the transmit PSD, S(f,lk), must be extremely high for the above relationship to hold. The received signal power for the short loops will be considerably higher than for long loops and, as a result, the SNR (and data rate) distribution as a function of loop length is similar to that of FIG. 3. However, the imbalance between the data rate on short loops compared with the data rate on long loops is even higher in the reference length method.
Like the reference length method and the equalized FEXT method, other power back-off methods provide for a SNR that varies with the loop length. Consequently, none of the prior methods provide a substantially equal data rate for all loop lengths. Nor do these methods teach how to control the upstream SNR, and consequently, upstream data rates in a VDSL environment. Furthermore, none of the prior methods provide for two (or more) data rate service areas based on differing transmit PSDs.